The present invention relates to the field of catalytic oxidation of hydrocarbons. More particularly, the present invention relates to the catalytic partial oxidation of paraffinic hydrocarbons, such as ethane, propane, and naphtha, to produce olefins, such as ethylene and propylene.
Olefins find widespread utility in industrial organic chemistry. Ethylene is needed for the preparation of important polymers, such as polyethylene, vinyl plastics, and ethylene-propylene rubbers, and important basic chemicals, such as ethylene oxide, styrene, acetaldehyde, ethyl acetate, and dichloroethane. Propylene is needed for the preparation of polypropylene plastics, ethylene-propylenc rubbers, and important basic chemicals, such as propylene oxide, cumene, and acrolein. Isobutylene is needed for the preparation of methyl tertiary butyl ether. Long chain mono-olefins find utility in the manufacture of linear alkylated benzene sulfonates, which are used in the detergent industry.
Low molecular weight olefins, such as ethylene, propylene, and butylene, are produced almost exclusively by thermal cracking (pyrolysis/steam cracking) of alkanes at elevated temperatures. An ethylene plant, for example, typically achieves an ethylene selectivity of about 85 percent calculated on a carbon atom basis at an ethane conversion of about 60 mole percent. Undesired coproducts are recycled to the shell side of the cracking furnace to be burned, so as to produce the heat necessary for the process. Disadvantageously, thermal cracking processes for olefin production are highly endothermic. Accordingly, these processes require the construction and maintenance of large, capital intensive, and complex cracking furnaces. The heat required to operate these furnaces at a temperature of about 900xc2x0 C. is frequently obtained from the combustion of methane which disadvantageously produces undesirable quantities of carbon dioxide and nitrogen oxides. As a further disadvantage, the crackers must be shut down periodically to remove coke deposits on the inside of the cracking coils.
Catalytic processes are known wherein paraffinic hydrocarbons are oxidatively dehydrogenated to form mono-olefins. In these processes, a paraffinic hydrocarbon is contacted with oxygen in the presence of a catalyst consisting of a platinum group metal or mixture thereof deposited on a ceramic monolith support in the form of a honeycomb. Optionally, hydrogen may be a component of the feed. The catalyst, prepared using conventional techniques, is uniformly loaded throughout the support. The process can be conducted under autothermal reaction conditions wherein a portion of the feed is combusted, and the heat produced during combustion drives the oxidative dehydrogenation process. Consequently, under autothermal process conditions there is no external heat source required. Representative references disclosing this type of process include the following U.S. Pat. Nos. 4,940,826; 5,105,052; and 5,382,741. A similar process is taught, for example, in U.S. Pat. No. 5,625,111, wherein the ceramic monolith support is in the form of a foam, rather than a honeycomb. Disadvantageously, substantial amounts of deep oxidation products, such as carbon monoxide and carbon dioxide, are produced, and the selectivity to olefins remains too low when compared with thermal cracking. As a further disadvantage, with prolonged use at high temperatures, the ceramic honeycomb and foam monoliths are subject to catastrophic fracture.
C. Yokoyama, S. S. Bharadwaj and L. D. Schmidt disclose in Catalysis Letters, 38, 1996, 181-188, the oxidative dehydrogenation of ethane to ethylene under autothermal reaction conditions in the presence of a bimetallic catalyst comprising platinum and a second metal selected from tin, copper, silver, magnesium, cerium, lanthanum, nickel, cobalt, and gold supported on a ceramic foam monolith. The use of a catalyst comprising platinum with tin and/or copper results in an improved olefin selectivity; however, the ceramic foam monolith is still prone to catastrophic fracture.
In view of the above, it would be desirable to discover a catalytic process wherein a paraffinic hydrocarbon is converted to an olefin in a conversion and selectivity comparable to commercial thermal cracking processes. It would be desirable if the catalytic process were to produce small quantities of deep oxidation products, such as carbon monoxide and carbon dioxide. It would also be desirable if the process were to achieve low levels of catalyst coking. It would be even more desirable if the process could be easily engineered without the necessity for a large, capital intensive, and complex cracking furnace. Finally, it would be most desirable if the catalyst was stable and the catalytic support not prone to fracture.
In one aspect, this invention is a process for the partial oxidation of a paraffinic hydrocarbon to form an olefin. The process comprises contacting a paraffinic hydrocarbon with oxygen in the presence of a catalyst. The contacting is conducted under autothermal process conditions sufficient to form the olefin. The catalyst employed in the process of this invention comprises at least one Group 8B metal supported on a fiber monolith support. Optionally, the catalyst may additionally comprise at least one promoter metal.
The process of this invention efficiently produces olefins, particularly mono-olefins, from paraffinic hydrocarbons and oxygen. In preferred embodiments, the process of this invention achieves a higher paraffin conversion and a higher olefin selectivity as compared with prior art catalytic, autothermal processes. Accordingly, in preferred embodiments, the process of this invention produces fewer undesirable deep oxidation products, such as carbon monoxide and carbon dioxide, as compared with prior art catalytic, autothermal processes. Even more advantageously, in preferred embodiments, the process of this invention achieves a paraffin conversion and olefin selectivity which are comparable to commercial thermal cracking processes. As a further advantage, the process produces little, if any, coke, thereby substantially eliminating problems with coking. Most advantageously, the process of this invention allows the operator to employ a simple engineering design and eliminates the requirement for a large, expensive, and complex furnace, as in thermal cracking processes. More specifically, since the residence time of the reactants in the process of this invention is on the order of milliseconds, the reaction zone used in this process operates at high volumetric throughput. Accordingly, the reaction zone measures from about one-fiftieth to about one-hundredth the size of a commercially available steam cracker of comparable capacity. The reduced size of the reactor reduces costs and greatly simplifies catalyst loading and maintenance procedures. Finally, since the process of this invention is exothermic, the heat produced can be harvested via integrated heat exchangers to produce energy, for example, in the form of steam credits, for other processes.
In another aspect, this invention is a catalyst composition comprising at least one Group 8B metal and at least one promoter metal, said metals being supported on a fiber-monolith support.
The aforementioned composition is beneficially employed as a catalyst in the autothermal partial oxidation of a paraffinic hydrocarbon to an olefin. In preferred embodiments, the catalyst composition beneficially produces the olefin at conversions and selectivities which are comparable to those of industrial thermal cracking processes. As another advantage, the catalyst composition of this invention exhibits good catalyst stability. Additionally, the fiber monolith support which is used in the composition of this invention can be advantageously manufactured into a variety of configurations, such as, without limitation, planar, tubular, and undulating configurations, for specific beneficial results, such as, to maximize the contacting conditions of the reactants with the catalyst and to minimize the pressure drop across the catalyst. As a further advantage, when deactivated the catalyst is easily removed from the reactor and replaced. Most advantageously, the fiber monolith support which is used in the catalyst of this invention is not prone to fracture as are the prior art honeycomb and foam monoliths.
In yet another aspect, this invention is a method of synthesizing or regenerating a catalyst on-line in an autothermal process of oxidizing a paraffinic hydrocarbon to an olefin. For the purposes of this aspect of the invention, the catalyst comprises a Group 8B metal and, optionally, a promoter metal on a monolith support. The term xe2x80x9con-linexe2x80x9d means the monolith support, either blank or in the form of a fully deactivated or partially deactivated catalyst, is loaded in the reactor and operating under ignition or autothermal process conditions. A xe2x80x9cblankxe2x80x9d support is a fresh support absent any Group 8B and, optional, promoter metals. The synthesis/regeneration method comprises contacting the front face of a monolith support with a Group 8B metal compound and/or a promoter metal compound, the contacting being conducted in situ under ignition conditions or autothermal process conditions.
The aforementioned method beneficially allows for the synthesis of an oxidation catalyst on-line under ignition conditions. Additionally, the aforementioned method beneficially allows for the regeneration of a deactivated or partially deactivated oxidation catalyst on-line under autothermal conditions. The method of this invention eliminates the necessity of preparing the catalyst prior to loading a reactor and eliminates the necessity of shutting down the reactor to regenerate or replace the deactivated catalyst. As a further aspect of this invention, novel catalyst compositions can be prepared and screened on-line for catalytic activity. The regeneration process can be beneficially employed on-line to replace metal components of the catalyst which are lost over time through vaporization. Dead sections of the catalyst can be reactivated or regenerated on-line. The aforementioned advantages simplify the handling and maintenance of the catalyst, reduce costs, and improve process efficiency.
The aforementioned on-line method of preparing or regenerating catalysts for autothermal processes produces catalysts in which the active catalytic components are selectively deposited on the front face of the monolith support. Thus, in another aspect, this invention is a catalyst composition comprising at least one Group 8B metal and, optionally, at least one promoter metal, said metal(s) being supported on the front face of a monolith support.
The catalyst composition, described hereinabove, is characterized by front face loading of the Group 8B element(s) and promoter element(s) onto the monolith support. This catalyst can be employed in the partial oxidation of a paraffinic hydrocarbon to an olefin under autothermal process conditions. Catalysts which are front face loaded advantageously exhibit improved activity in these oxidation processes, as compared with catalysts characterized by uniform loading throughout the support.
In yet another aspect, this invention is a second process of partially oxidizing a paraffinic hydrocarbon to an olefin. The process comprises contacting a paraffinic hydrocarbon with oxygen in the presence of a catalyst under autothermal process conditions. The catalyst used herein comprises at least one Group 8B metal and, optionally, at least one promoter metal, said metal(s) being loaded onto the front face of a monolith support.
The aforementioned second autothermal oxidation process employs a catalyst characterized by front face loading of the Group 8B element(s) and optional promoter element(s) onto a monolith support. This second autothermal oxidation process enjoys all of the benefits of the first autothermal oxidation process employing fiber monolith supports, described hereinbefore. More advantageously, the process of this invention characterized by front face loading of the catalyst results in a higher paraffin conversion and a higher olefin selectivity, as compared with catalysts having uniform loading throughout the support.